The exposure of small substrates such as those used for plasma display screens is generally performed with both the substrate and the photo tool or mask in a horizontal position by a single exposure from a vertically directed collimated light beam. However, for a number of reasons, such methods do not work well for large panels such as those used for large plasma display television screens. One problem is that for a large substrate, it is difficult to provide a collimated light beam of a sufficient size to permit the single exposure of the substrate at a uniform intensity over its entire surface. Another problem is that when a large mask is held in a horizontal mask frame for exposure of a substrate, the center of the mask tends to sag due to its weight. Similarly, unless it is properly supported, a horizontal substrate will tend to sag. Any such sagging in the mask or substrate makes the accurate photolithography of the substrate virtually impossible.
While a number of manufacturing techniques have been developed in attempts to avoid these problems, such techniques have proven to be difficult and costly to implement. Moreover, current methods do not generally permit the quick and accurate exposure of a large number of large substrates in an automated fashion.
Generally, the methods for exposing large substrates have required either scanning the entire surface of the panel with a moving beam and/or using a fixed beam and moving the mask and substrate so as to expose the substrate's entire surface by a scanning procedure. These scanning methods include the "step and repeat" method in which specific portions of the panel are sequentially exposed in incremental steps, each step by a timed exposure. Other methods include a continuous scan in which the substrate and light beam are moved with respect to one another in a moving exposure. One such continuous scan method moves the mask and substrate over a serpentine path to expose the substrate to a fixed collimated light beam in a single exposure until the entire substrate has been exposed.
Both step and repeat and continuous scan methods of exposure often lead to stitching errors, that is, errors where discrete lines of overexposure or underexposure are left in the substrate where adjacent paths of the collimated light beam do not perfectly mesh with one another. To the extent stitching errors can be reduced, these basic methods are nonetheless time consuming and often difficult to consistently repeat for a large batch of substrates. Problems with repeatability lead to the frequent rejection of substrates for failing to meet minimum quality control standards.
With regard to the problem of mask sagging when a large mask is held in a horizontal position for exposure of a substrate using a vertical light beam, this problem has largely been overcome through the use of a generally horizontal beam of exposure light that is directed through a mask and substrate held in a generally vertical orientation. However, when the mask and substrate are held perfectly vertical, they tend to be unstable and difficult to hold motionless during the exposure. The stability of the mask and substrate can be improved while reducing sag by holding the mask and substrate in planes that are slightly out of vertical. Nevertheless, while sag problems can generally be avoided by these methods, the handling of large, vertically oriented masks and substrates has proven to be difficult. These problems in handling make conventional exposure methods for large substrates costly, time consuming and unreliable.
Yet another difficulty with existing methods for exposing large substrates has been that such exposures must generally be done in a clean room so as to avoid contaminating either the mask or the photoresist coating on the substrate with dust or dirt. Because the equipment associated with the exposure of large substrates is very large, it is often difficult and costly to fit such equipment within a clean room. Moreover, once the equipment is installed in a clean room, any routine maintenance can be difficult and costly due to the steps that must be taken in maintaining the clean room atmosphere.
An improved apparatus and method for exposing large photoresist-coated substrates quickly, reliably and at low cost is desired.